1. Field of the Invention
This invention relates to a pickup assembly for use in an apparatus for reading optically an information recorded on a disc as a track or tracks.
2. Description of the Prior Art
A disc is known on which digitalized informations such as video signals or audio signals are recorded along a spiral track in the form of pits. The length of the pit and the distance between the pits vary with the content of the information. In a disc player for reproducing the disc, for example, an optical contactless pickup is used for detecting the informations. The pickup radiates laser on to the surface of the disc, and receives the reflected laser from the surface of the disc at its light-sensing element. The reflected laser is modulated due to diffraction phenomenon by the pits. The pickup is attached to a pickup arm which is lenearly moved or rotatably supported. In the reproducing operation, the pickup is moved towards the center of the disc in the radial direction, or moved outwards from the center of the disc, facing to the disc, so that the spiral track is traced by the pickup.
In the disc player with the contactless pickup, there are made a tracking error of the pickup, a time base error and a focus error during the reproducing operation due to eccentricity of the disc, warp thereof and external vibration. In the pickup, an optical system is so arranged as to be movable in the X-axis, Y-axis and Z-axis and to control the radiation position of the laser and the focus point thereof for correcting the errors. A movable member in the optical system is moved in the radial direction of the disc, namely along the X-axis, so as to correct the tracking error. It is moved in the normal direction to the disc, namely along the Z-axis, so as to correct the focus error. And it is moved in the direction of the track formed on the disc, namely along the Y-axis, so as to correct the time base error.
The U.S. Pat. No. 4,092,529 discloses a method to move the movable member along the X-axis, Y-axis and Z-axis. In the method, a comination of coil and magnet is used. Coils are wrapped on a tube including optical system, and magnets are so arranged as to intersect with the coils. An error signal current is supplied to the coils to move the entire tube and control the optical path, so that the tracking error and the time base error are corrected. Further, a cylindrical member is arranged on the tube, and a coil is wrapped on the cylindrical member, facing to a magnet. An error signal current is supplied to the coil to drive the tube up and down, so that the focus error is corrected. In such a device, the movable member is constituted by many parts. Accordingly, the device is complicated in construction, and heavy. Although the errors should be corrected at high speed, the movable member as a servo system is inferior in response. Further, the movable member has a proper resonant frequency. Accordingly, when the frequency of the movement of the movable member by the servo signal coincide with its resonant frequency, the servo function is lost.